Multivibrator



C. R. KENNY MULTIVIBRATOR June 12, 1962 Filed Dec. 22, 1960 l, OUTPUT JJ OUTPUT T|ME j INVENToR. CHARLES R. KENNY BY 70@ ATTORNEY.

United States Patent O 3,639,066 MULTIVIBRATQR Charles R. Kenny, Purdy Station, NSY., assignor to General Precision, line., a corporation of Deiaw'are Filed Dec. 22, 1960, Ser. No. 77,704 6 Claims. (Cl. 331-113) This invention relates to astable multivibrators and more particularly to such multivi-brators employing two transistors with their emitter-col-lector paths in series.

The usual transistor multivibrator circuit employs two transistors with their collector-emitter paths in shunt between power supply terminals, and with bases cross-'connected to the collectors. Such a multivibrator circuit output is not grounded, and the usual way of securing a symmetrical output waveform is by taking the output between the two collector terminals. There is no way of securing a symmetrical waveform between any single point of the circuit and ground.

The present circuit differs over former circuits of this type in providing an output terminal at which there is an inherently symmetrical potential output waveform when measured with respect to ground. In addition, the present circuit provides an approximately triangular output by reason of an inherent integrating action. By adding additional components, the output can be made more nearly triangular.

The inventive circuit places two transistors in series, with a resistor connecting the emitter of the tirst to the collector of the second. The collector of the iirst is connected to battery and the emitter of the second is grounded. The transistors can be either both PNP or both NPN, with appropriate bias `potentials and polarities. Positive feedback loops, each containing a capacitor, are connected from the base of each to the collector of the other. The output is taken from the emitter of the rst transistor. It is essential to connect a suitabie capacitance between the output terminal and ground. At the output a relatively small peak-to-peak voltage output is secured having a triangular waveform composed of exponential segments. By adding a series resistor and shunt capacitance of the correct dimensions an approximately straight-line symmetrical triangular output is obtained.

One object of this invention is to provide a free-running multivibrator circuit which produces a symmetrical, single-ended output between an output terminal and a grounded or common terminal.

Another object of this invention is to provide a transistor multivibrator oscillator emitting symmetrical, substantially triangular Waveform, alternating current in a circuit having one side common, or at ground potential.

A further understanding of this invention can be secured from the detailed description and drawing, in which:

FIGURE l is a schematic diagram of one embodiment of the invention,

Fl-GURES 2 and 3 are graphs of the output waveforms of the circuit of the invention.

Referring now to FGURE 1, an NPN transistor 11 has its collector 12 connected to a source of positive direct current at terminal 13 through a resistor 14. The emitter 16 is connected through a resistor 17 to the collector 18 of another, similar, NPN transistor 19. lts emitter 21 is grounded. A positive feedback connection through a capacitor 22 and a base current limiting resistor 23 connects collector 18 and base 24. Another, similar positive feedback branch consisting of capacitor 26 and a base current limiting resistor 27 connects collector 12 to base 28. The transistor bases are protected against eX- cessive inverse currents by diodes 29 and 31. The base 28 Mice is biased through resistor 32 from a point of about +3 volts relative to its emitter and ground, that point being the junction of the divider composed of resistor 33 and potentiometer 34, connected to the positive potential terminal 13 and ground. The base 24 is biased through resistor 36 from a point of about 14 volts relative to ground, or a point of about 3 volts relative to its emitter, that point being the junction o1 the divider composed of resistor 37 and potentiometer 34. A capacitor `318 is connected between the emitter 16 and ground. A primary output terminal 39 is connected to the emitter 16. A secondary output terminal 41 is connected to the terminal 39 by the series resistor 42 and shunt capacitor 4G.

Suitable component values for a frequency of oscillation of about 5() c.p.s. are:

Resistor 37 13K ohms. Resistor 14 2.7K ohms. Resistor 33 39K ohms. Resistor 36 100K ohms. Resistor 23l i 3.3K ohms. Resistor 17 2.7K ohms. Resistor 27 3.3K ohms. Resistor 32 100K ohms. Resistor 42 1000 ohms. Potentiometer 34 50K ohms. Capacitor 22 .47 mfd. Capacitor 26 .47 mfd. Capacitor 38` 4 mfd. Capacitor 43 4 mfd. Supply terminal 13 +22 volts. Transistors 11 and 19 Type 2N338 or similar.

The function of potentiometer 34 is to balance the direct current base bias potentials so that the two base-toemitter biases will be equal. The slider 45 should therefore be adjusted toward the base 28 to provide a potential of about +3 volts at this base relative to the emitter 21. Similarly, the potential at base 24 is then about +14 volts relative to ground, providing about 3 volts bias relative to the emitter 16.

In steady state operation, assume that transistor 19 is conductive and transistor 11 nonconductive. Capacitor 38 is in the charged condition, with its upper terminal positive by about 13 volts relative to ground. The output waveform as it would appear on an oscilloscope connected to terminal 39 is shown in FIGURE 2. Under the assumed steady state conditions, operation begins at point 44. At point 44 transistor 19 begins conduct-ing, and transistor 11 stops conducting. The capacitor 3S ybegins to discharge through resistor 17 and transistor 19. The potential drop created in resistor 17 causes the base 24 of transistor 11 to be more negative than emitter 16, so that transistor 11 is held in a nonconductive condition. The upper terminal of capacitor 26 begins to charge toward +22 volts, making the base 28 more positive and holding transistor 19 iirmly conductive.

The upper terminal of capacitor 22 and the lower terminal of capacitor 26 now commence to discharge through their resistors, principally resistors 36 and 32, toward their steady-state bias potentials, while at the same time the charging currents into their opposite terminals decrease. In addition, the potential of terminal 39 and of emitter 16 follows the curve 46, FIGURE 2, toward the potential of +9 volts at point 47.

At this point the potential of base 28 has decreased enough to reduce the current through transistor 19 while the potential dierence between base 24 and emitter 16 h-as become about zero and the transistor 11 begins to draw current. These conditions reverse the current llow at capacitor 38, which lbegins to charge through transistor 11 from the positive terminal 13. Since both of the feedback loops, through capacitors 22 and 26, are regenerative, these changes, once started, go to their conclusions abruptly `at point 47, causing transistor il to become conductive and 19 to become nonconductive.

The capacitor 38 now charges `along the curve 48, FIGURE 2. The sudden drop of col-lector 12 to about +10 volts produces a negative pulse through capacitor 26 holding the base 28 negative, and the rise in potential of collector 18 increases the current into base 24 which holds transistor 11 fully conductive. Thus the cycles shown in FIGURE 2 are repeated and the circuit maintains its oscillation.

The addition of capacitor 43 and resistor 42 tend to bring the exponentially triangular output of terminal 39 toward a linear triangular form by adding an exponential component. By judicious selection of values, one set of which is given above, it is possible to produce a reasonably linear output lat terminal 41 similar to that shown in FIGURE 3.

It is to be noted that the output waveforms are inherently symmetrical in the sense that the negative halt cycles are identical with the positive half cycles inverted.

The circuit of FIGURE l has a reasonably exact vacu um tube triode analog. If the transistors be replaced by tubes and the plate potential suitably changed, the circuit will oscillate and have the described operation, features and advantages. Suitable `changes of component values and diierent fixed biases will be desirable.

What is claimed is:

1. An astable multivibrator comprising, a pair of transistors each having a base, collector and emitter, a resistor joining the collector of one to the emitter of the other, resistive means joining the remaining collector to a terminal of a potential source having its opposite terminal connected to ground, a connection from the remaining emitter to ground, a capacitor and resistor connected in series joining the base of one transistor to the collector of the other, a second capacitor and second resistor connected in series joining the base of the other transistor to the collector of said one transistor, and a capacitor coupling the junction of said resistor and emitter to ground, said junction serving as one output terminal and ground as the other.

2. An astable multivibrator comprising, a pair of transistors each having base, collector and emitter, a first resistor joining the collector of one of said pair of transistors to the emitter of the other, a second resistor joining the collector of said other transistor to the terminal of a battery, the other battery terminal being grounded, a connection from the emitter of said one transistor to ground, a tirst capacitor and resistor connected in Series joining the base of said one transistor to the collector of the other, a second capacitor and resistor in series joining the base of said other transistor to the collector of the one, means applying biases to said bases, and capacitive means coupling junction of said iirst resistor and emitter to ground, said junction serving with said ground as output terminal whereby a symmetrical voltage waveform to ground is generated.

3. An astable multivibrator comprising, a pair of transistors each of which includes at least a base, collector and emitter, said transistors having their collectoremitter paths connected in series with each other and across a source of direct current supply one terminal of which is grounded, a series resistor inserted in series with said collector-emitter paths and between the collector of one transistor and the emitter of the other, a pair of positive feedback paths cross coupling the collectors and bases of said transistors, a capacitor connected between ground and the common junction of said series resistor and transistor emitter, and unitary means lfor adjusting the base-emitter biases of said transistors.

4. An astable multivibrator as set forth in claim 3 having a lter output circuit connected in shunt to said capacitor.

5. An astable multivibrator comprising, a pair of transistors each of which includes at least a base, collector and emitter, said transistors having their collector-emitter paths connected in series with each other through an intermediate resistor connected 'between the emitter of one transistor and the collector of the other, a source of direct current supply having one potential terminal connected to the collector of one of said transistors and a second potential terminal connected to the emitter of the other of said transistors, a pair of positive feedback paths each comprising a capacitor and resistor connected in series cross coupling the collectors and bases of said transistors, a capacitor connected between the junction of said intermediate resistor with the emitter of said one transistor and said second potential terminal, unitary means for adjusting the base-emitter biases of said transistors, and an output circuit connected across said last named capacitor. v

6. An astable multivibrator as set forth in claim 5 in which said means yfor adjusting the base-emitter biases comprises a potentiometer having its end terminals each connected through a resistor to said one terminal of said direct current source and a slider on said potentiometer connected to said second potential terminal, the respective end terminals of said potentiometer being connected to the respective base terminals of said transistors.

No references cited. 

